Why don’t we just move the age of adulthood to 21?

So one of the suggestions for gun control is raising the age to be able to by a rifle to 21. I’m not necessarily against the proposal. But I’m confused on why you should be able to to be drafted in the army or have the ability to vote or even go to jail for life if your not able to smoke drink or buy a gun. Adulthood is adulthood either your a responsible adult at 18 and everything that comes with being an adult come with it the ability to smoke drink vote join the military or go to prison for life and be treated as an adult at 18 or we could do all that at 21 yes this means you have 3 extra years of being a kid after you graduate high school
Why don’t we just move the age of adulthood to 21?
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